linux-kernel - Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless

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Date:	Thu, 14 Mar 2013 17:24:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>, Shaohua Li <shli@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: +
	atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
	.patch added to -mm tree

> From: Ming Lei <tom.leiming@...il.com>
> Subject: atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive
>
> Generally, both atomic_inc_unless_negative() and
> atomic_dec_unless_positive() need at least two atomic_cmpxchg() to
> complete the atomic operation.  In fact, the 1st atomic_cmpxchg() is just
> used to read current value of the atomic variable at most times.

Agreed, this looks ugly...

But can't we make a simpler patch and keep the code simple/readable ?

Oleg.

--- x/include/linux/atomic.h
+++ x/include/linux/atomic.h
@@ -64,7 +64,7 @@ static inline int atomic_inc_not_zero_hi
 static inline int atomic_inc_unless_negative(atomic_t *p)
 {
 	int v, v1;
-	for (v = 0; v >= 0; v = v1) {
+	for (v = atomic_read(p); v >= 0; v = v1) {
 		v1 = atomic_cmpxchg(p, v, v + 1);
 		if (likely(v1 == v))
 			return 1;
@@ -77,7 +77,7 @@ static inline int atomic_inc_unless_nega
 static inline int atomic_dec_unless_positive(atomic_t *p)
 {
 	int v, v1;
-	for (v = 0; v <= 0; v = v1) {
+	for (v = atomic_read(p); v <= 0; v = v1) {
 		v1 = atomic_cmpxchg(p, v, v - 1);
 		if (likely(v1 == v))
 			return 1;

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